1. Field of the Invention
The present invention relates to a photothermal conversion measurement apparatus and a photothermal conversion measurement method used in a process of analyzing, for example, a substance contained in a liquid sample to measure a property change of the sample based on a change in the refractive index of the sample that is caused by a photothermal effect when the sample is irradiated with excitation light. The present invention also relates to a sample cell that contains the liquid sample to be analyzed.
2. Description of the Related Art
In the analysis of substances contained in samples, such as various kinds of liquid samples, it is important to increase the analytical sensitivity in order to reduce the amounts of reagents, to simplify the process of concentrating the samples, to improve the analytical efficiency, and to reduce costs.
On the other hand, when a portion of a sample is irradiated with excitation light, the irradiated portion absorbs the excitation light and generates heat. This is called a photothermal effect, and a measurement of the thus generated heat is called a photothermal conversion measurement.
As an example of a known high-sensitivity analysis method by which a sample is analyzed using the photothermal conversion measurement, a method that uses a thermal lens effect that occurs in the sample due to the photothermal effect (hereafter called a thermal lens method) is known.
An analysis apparatus using the thermal lens method (photothermal conversion spectroscopic analysis apparatus) is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-232210. In this apparatus, detection light (measurement light) is caused to be incident on a sample and is collected such that the detection light passes through a pinhole. The intensity of the detection light that comes out from the pinhole is detected, and accordingly a change in the refractive index of the sample caused by heat generated by the sample when excitation light is incident on the sample is detected as a change in the state in which the detection light is collected.
On the other hand, according to Japanese Unexamined Patent Application Publication No. 2004-301520, a change in the refractive index of a sample caused by a photothermal effect thereof is determined by measuring a phase change of measurement light that passes through (that is transmitted by) the sample by optical interferometry.
Accordingly, even if the position of a photodetector (photoelectric conversion means), the intensity of the measurement light, and the intensity distribution of the measurement light differ for each apparatus, a change in the refractive index of the sample can be stably measured with high optical accuracy and sensitivity without being influenced by such factors as long as they do not vary during the measurement. This solves the problems of the above-described thermal lens method.
On the other hand, Japanese Unexamined Utility Model Registration Application Publication No. 5-23072 discloses a technique for measuring a spectrum of a Fourier interference pattern obtained by a Fourier spectrometer with a high dynamic range.
In addition, Japanese Unexamined Patent Application Publication No. 2000-356611 discloses a high-sensitivity analysis method using the photothermal conversion measurement as an in-situ analysis technique that meets the needs for high-speed, high-sensitivity detection of small amounts of biomolecules in a microchannel. The analysis using the photothermal conversion measurement is known to be effective in detecting, for example, a small amount of molecules after a measured substance is separated by chromatography, and various improvements have been suggested to increase the analysis speed and sensitivity.
FIG. 17 is a schematic diagram illustrating a photothermal conversion measurement apparatus jXO as an example of a known photothermal conversion measurement apparatus. The photothermal conversion measurement apparatus jXO will be explained below with reference to FIG. 17.
As shown in FIG. 17, the photothermal conversion measurement apparatus jXO includes an excitation light source j1, a chopper j2, a dichroic mirror j3, a lens j4, a measurement light source j6, a half-wave plate j7, a mirror j22, a polarizing beam splitter (PBS) j8, acousto-optical modulators j9 and j10, mirrors j11 and j12, polarizing beam splitters (PBS) j13 and j14, a mirror j15, quarter-wave plates j16 and j18, a photodetector j17, a reflection mirror j19, a polarizing plate j20, and a signal processor j21. A sample cell j5 containing a sample to be analyzed is disposed at a predetermined position. The sample cell j5 contains the sample to be analyzed, and is filled with, for example, a sample dissolved in a solvent. In the following description, if a solvent is contained in the sample cell, it is to be considered that the solvent is included in the sample cell.
The excitation light source j1 emits excitation light E. The excitation light source j1 is, for example, a laser with a wavelength of 533 nm and an output of 100 mW (yttrium aluminum garnet (YAG) harmonic wave). The excitation light E is converted into chopped light by a predetermined period by the chopper j2. Then, the excitation light E is reflected by the dichroic mirror j3, passes through the lens j4, and is incident on the sample contained in sample cell j5. The sample contained in the sample cell j5 absorbs the excitation light E and generates heat (photothermal effect), and the thus generated heat is absorbed by the solvent. Accordingly, the refractive index of the sample changes.
Measurement light M for measuring the change in the refractive index of the sample is emitted by the measurement light source j6. The measurement light source j6 is, for example, a He—Ne laser with an output of 1 mW. The measurement light M enters the half-wave plate j7, where the plane of polarization of the measurement light M is adjusted, and is reflected by the mirror j22. Then, the measurement light M is divided by the PBS j8 into two polarized waves M1 and M2 that are perpendicular to each other.
The polarized waves M1 and M2 enter the acousto-optical modulators j9 and j10, respectively, and are frequency-converted. At this time, the frequencies of the polarized waves M1 and M2 are set so as to differ from each other by, for example, 30 MHz. Then, the polarized waves M1 and M2 are reflected by the mirrors j11 and j12, respectively, and are combined by the PBS j13.
The polarized wave M2 in the combined wave passes through (is transmitted by) the PBS j14, is reflected by the mirror j15, and enters the PBS j14 again. Since the polarized wave M2 passes through the quarter-wave plate j16 disposed between the PBS j14 and the mirror j15 twice, the plane of polarization thereof is rotated by 90°. The polarized wave M2 is reflected by the PBS j14 toward the photodetector j17.
The polarized wave M1 in the combined wave is reflected by the PBS j14, passes through the quarter-wave plate j18, the dichroic mirror j3, and the lens j4, and is incident on the sample in the sample cell j5. In this apparatus, the polarized wave M1 and the excitation light are incident on the sample at the same irradiation position in the sample cell j5.
The polarized wave M1 passes through the sample cell j5, is reflected by the reflection mirror j19, and returns to the PBS j14 along the same optical path as the optical path along which the polarized wave M1 travels to the sample cell. Since the polarized wave M1 passes through the quarter-wave plate j18 twice, the plane of polarization thereof is rotated by 90°. Accordingly, the polarized wave M1 is combined with the polarized wave M2 when the polarized wave M1 passes through the PBS j14 and the combined light travels toward the photodetector j17.
The polarizing plate j20 is disposed between the PBS j14 and the photodetector j17. In the polarizing plate j20, the polarized wave M1 and the polarized wave M2 interfere with each other as measurement light and reference light, respectively.
The photodetector j17 detects the interference light obtained by the polarized waves M1 and M2 and outputs an electric signal representing the intensity of the interference light to the signal processor j21.
The intensity of the interference light varies depending on a phase change when the polarized wave M1, which functions as the measurement light, passes through the sample cell j5. Accordingly, the phase change of the polarized wave M1, which functions as the measurement light, can be determined on the basis of the measurement result of the intensity of the interference light. Thus, a change in the refractive index of the solvent filling the sample cell j5 can be determined.
In each of the structures described in Japanese Unexamined Patent Application Publication Nos. 10-232210 and 2004-301520, substantially the entire path along which the measurement light travels in the sample is excited by the excitation light and an average property of the entire path of the measurement light in the sample is measured. Therefore, a property distribution in the sample, in particular, a property distribution along the depth of the sample from the surface thereof cannot be obtained.
When the absorption spectral property of the sample is to be evaluated, a white light source is used as the excitation light source. White light emitted from the white light source is divided into spectral components and the measurement is performed each time the wavelength range of the excitation light is changed. A typical white light source generally has a wide light-emitting section, and therefore it is difficult to collect light emitted from the white light source with high accuracy before irradiating the sample with the light.
However, the white light source cannot be used in the measurement using the thermal lens method described in Japanese Unexamined Patent Application Publication No. 10-232210, since the excitation light must be collected with high accuracy before irradiating the sample therewith in order to obtain the thermal lens effect. Therefore, a laser oscillator with a specified wavelength range must be used and it is difficult to evaluate the absorption spectral property of the sample. Although the absorption spectral property of the sample can, of course, be evaluated by the thermal lens method if a plurality of laser oscillators with different wavelength ranges or a laser oscillator with a variable wavelength range is used, the structure of the apparatus becomes complex and the cost is increased in such a case.
In addition, in the measurement using the thermal lens method described in Japanese Unexamined Patent Application Publication No. 10-232210, the intensity of the excitation light must be increased or the diameter of the pinhole through which the measurement light travels after passing through the sample must be reduced in order to increase the measurement sensitivity. However, if the intensity of the excitation light is increased, electric power consumption and cost are also increased. In addition, if the pinhole diameter is reduced, a signal-to-noise (S/N) ratio is reduced due to a reduction in the amount of light received by the photodetector and the measurement time is increased.
In addition, in the measurement described in Japanese Unexamined Patent Application Publication No. 2004-301520, since an optical interferometer, in which a relatively large number of optical devices must be arranged with high positioning accuracy, is used, the structure of the apparatus is complex. In addition, although the optical interferometer is largely influenced by disturbance noise, such as vibration, it is difficult to reduce the disturbance noise.
The accuracy of the analysis depends on the intensity of a detection signal that varies in accordance with the temperature change of the solvent. Therefore, it is desirable to obtain a high-intensity detection signal to increase the analysis accuracy.
To obtain a high-intensity detection signal, it is necessary to cause the sample to generate a large amount of heat. In other words, it is necessary to set the intensity of the excitation light incident on the sample as high as possible. However, if a high-intensity (high-brightness) light source is used as the excitation light source, electric power consumption and cost are increased.
When the sample contained in the sample cell is measured, members other than the sample, such as a container member of the sample cell and the solvent enclosed therein, are also irradiated with the excitation light E emitted from the excitation light source j1. Therefore, if the wavelength range of the excitation light includes the absorption wavelength range of the container member or the solvent, a part of the excitation light is absorbed by the container member or the solvent, and accordingly the container member or the solvent generates heat.
FIGS. 18A to 18C are graphs showing examples of optical absorption properties of a measurement sample, a container member, and a solvent, respectively. In each graph, the horizontal axis shows the frequency of light (reciprocal of the wavelength of light) and the vertical axis shows the optical absorptivity. The sample is linseed oil and the solvent is chloroform.
As shown in FIG. 18A, the optical absorptivity of the sample (linseed oil) has peaks in frequency ranges of 1400 cm−1 to 1470 cm−1 and 1710 cm−1 to 1770 cm−1.
As shown in FIG. 18B, the container member efficiently absorbs light with a frequency range of 1100 cm−1 or less. Therefore, if the excitation light has a component with a frequency of 1100 cm−1 or less, the container member absorbs a part of the excitation light and generates heat.
In addition, as shown in FIG. 18C, light that is easily absorbed by the solvent (chloroform) has a frequency range of 1400 cm−1 to 1550 cm−1. Therefore, the solvent generates heat if the excitation light has a component with a frequency in the range of 1400 cm−1 to 1550 cm−1.
When the solvent of the sample or the container member generates heat as described above, the signal input to the signal processor j21 includes a noise signal generated due to heat generated by the members other than the sample. Thus, the heat generated by the members other than the sample causes a reduction in the measurement accuracy of the heat generated by the sample.
Accordingly, frequency components of the excitation light other than those with frequency ranges (wavelength ranges) that can be easily absorbed by the sample are preferably eliminated before the excitation light is incident on the sample. More specifically, in the example shown in FIGS. 18A to 18C, components with frequency ranges of 1100 cm−1 or less and 1470 cm−1 to 1550 cm−1 are preferably eliminated.